The optical fibre interferometric current sensor relates to the field of electrical current measurement. In particular, this sensor is used for measuring electrical current in high voltage conductors. Generally, according to actual technology, measurements of electrical current in high voltage conductors are done by means of current transformers. These apparatuses are very expensive. Also, they cannot be mounted easily. Furthermore, these current transformers can explode in case of dielectric breakdown. Whereas optical fibre sensors are safe and less expensive while providing technical characteristics which are superior to the ones provided by current transformers. Among these technical characteristics, it can be mentioned that they are provided with a dynamic range superior to the one obtained with current transformer, with pass-band superior to the one of the current transformers and with a precision which is as good as the one provided with current transformers or superior. Also, optical fibre sensors are very light, compact and made of dielectric material. Accordingly, they can be easily mounted.
There are two families of optical fibre sensors. The first comprises the bulk type sensors based on the use of a block of glass, and the second comprises fibre sensors using an optical fibre not only for guiding light energy as it is the case with the bulk sensor, but also for the measurement itself. Certain bulk sensors have been developed to a point where they have reached a pre-commercialization state, whereas the optical fibre sensors are generally less developed.
Nevertheless, development of optical fibre sensors continues because these have possibilities which are superior to the ones of the bulk type sensors. More specifically, in certain conditions they can provide better precision and reliability. Also, they can be produced at a lower cost and they can be used in a broader range of applications.
The use of an optical fibre can be beneficial by encircling the conductor with several turns, to multiply the Faraday rotation by the number of turns. This is true for an ideal optical fibre but for a non-ideal optical fibre, the Faraday rotation is quenched because of the linear birefringence of the optical fibre. This problem should be solved by minimizing the quenching caused by the linear birefringence. A solution consists in using a high-birefringence spun fibre.
Sensors using standard optical fibres have problems. They are very sensitive to mechanical vibrations and to temperature variations. Certain groups of researchers seem to have solved the problem relating to temperature variations whereas other groups seem to have solved the problem relating to sensitivity to mechanical vibrations, but nobody has succeeded in solving these two problems at the same time.
Most optical current sensors are based on a phenomenon known as the Faraday effect. When a light beam is in the proximity of a magnetic field oriented along a direction parallel to the light beam propagation direction, then the two circular components of its polarization state are subjected to a phase-shift proportional to the intensity of the magnetic field. The polarity of this phase-shift depends on the direction of the magnetic field with respect to the direction of the light beam propagation. It is said that this phenomenon is non-reciprocal.
Several configurations based on glass bulk have been proposed in the literature. In this case, a block of glass is shaped as a ring around the conductor, and the light guided inside the block is subjected to a polarization rotation which is proportional to the electrical current. Nevertheless, the Faraday rotation caused by the presence of current is very weak. Also, the coded light signal is very weak.
The phase difference produced between the circular polarization components produced by the Faraday effect induces, for a linear polarization, a rotation .PHI. of the polarization plan equal to the half of the phase difference 2.PHI. produced between the two circular polarization components. This is why this phenomenon is also known as the Faraday rotation. Along a propagation distance L, the phase difference .PHI. is calculated by means of the following equation: ##EQU1## where H is the magnetic field vector, .mu. is the permeability constant and V is the Verdet constant which is representative of the sensitivity of the medium to the Faraday effect. According to Ampere's law, the magnetic field integrated along a closed loop depends only on the current I circulating inside the loop where I is defined according to the following equation: EQU .intg..smallcircle.H.multidot.dl=I
Thus, by circulating an optical beam along a closed loop path around an electrical conductor by means of an optical fibre, it is possible to know the amplitude of the electrical current by measuring the phase difference 2.PHI. produced between the right and left circular components of the polarization according to the following equation: EQU .phi.=.mu.VNI
where N is the number of complete turns done by the light beam around the electrical conductor.
A certain degree of birefringence is always present in an optical fibre. This birefringence can be intrinsic or extrinsic. The intrinsic birefringence results from the characteristic of the fibre due to its manufacturing process. The extrinsic birefringence results from ambient conditions surrounding the fibre such as the temperature, the strain and the vibration. The linear birefringence present in the fibre produces a quenching of the phase difference between the circular polarization components (the Faraday rotation) resulting from the Faraday effect.
The optical fibre which will be used in the present invention is known in the art as a high birefringent spun optical fibre (spun fibre). Such a fibre has high linear and circular birefringences. Because of this double characteristic, this fibre is less sensitive to changes in ambient conditions than a low intrinsic birefringence fibre while maintaining a good sensitivity to electrical current. This fibre is known as spun fibre because during its manufacturing process, the preform is axially rotated during its drawing into a fibre.
However, the optical fibre itself is not sufficient for measuring electrical current with good results. Several configurations of optical fibre interferometric current sensor using the high-birefringence spun fibre have been experimented. The sensor has to be designed in such a way that the mechanical vibrations and the temperature variations do not affect the current reading. The Faraday rotation in a high-birefringence spun fibre is always present. Nevertheless, the linear and circular birefringences of the optical fibre also affect the polarization state of the light guided through the optical fibre. As the mechanical vibrations and the temperature variations affect the linear and circular birefringences, the polarization and the phase are also affected. From that, it is clear that the current reading will be dependent upon temperature variations and mechanical vibrations which can have a greater influence than the Faraday rotation itself. This is the problem to be solved.
This problem should be solved by taking into consideration a specific characteristic of the Faraday effect, the non-reciprocity. This means that the Faraday rotation of the light circulating through an optical fibre from point A to point B, and then inversely from point B to point A is cumulative as there is no reciprocity. On the other hand, the circular and linear birefringences are reciprocal. This means that rotation of the light circulating through an optical fibre from point A to point B, and then inversely from point B to point A is cancelled as there is reciprocity. Then, the effect of the circular and linear birefringences, under certain conditions is cancelled. From that, it is possible to obtain insensitivity to mechanical vibrations and temperature variations. These conditions are filled if the path of the light circulating in both directions is reciprocal, it means that both counter-propagating light beams follow similar reciprocal paths. It is believed that there is no configuration in the prior art that satisfies the condition mentioned above. With reciprocal paths, it is possible to compensate signal variations resulting from vibrations or mechanical strains applied to the fibre.
Several embodiments using the spun fibre have been proposed for measuring electrical current. In the International PCT application No. WO 83/00232 of PAYNE et al., the use of an orthoconjugated mirror disposed at the end of a spun fibre is suggested. The light is injected at an end, is guided along the fibre and is reflected again inside the fibre. The reflected light is detected after its return course along the fibre by means of a polarizing prism. The orthoconjugated mirror reflects the light so that it follows a reciprocal path in both directions along the fibre.
But, orthoconjugated mirrors, upon which depends the stability of the whole system, are themselves sensitive to temperature variations and to magnetic field. Accordingly, reciprocal paths using an orthoconjugated mirror do not solve the problems relating to temperature variations.
Also known in the art, is the Sagnac interferometer. In such interferometers, two counter-propagating light beams are injected into the two opposite ends of the same optical fibre by means of input/output ports of a coupler. Said optical fibre encircles the electrical conductor through which circulates the electrical current to be measured. This coupler is also used for recombining the counter-propagating light beams on the return thereof. If these counter-propagating light beams follow reciprocal paths, the phase difference induced by the optical fibre is nil. As the Faraday effect is non-reciprocal, the presence of a magnetic field will induce a positive phase difference between circular polarization components for a light beam circulating along a given direction, and a negative phase difference for the other light beam which is counter-propagating. Thus, the phase difference between the light beams, after recombination at the coupler, is dependent only upon the Faraday effect and therefore it is dependent upon the electrical current circulating through the conductor. A variation of the phase difference between the two counter-propagating light beams modifies the interfering conditions during the recombination at the coupler. Resulting from that is a variation of the light intensity distribution reflected in input/output ports of the coupler according to the following equations: EQU P.sub.1 =cos.sup.2 (.mu.VNI)P.sub.c EQU P.sub.2 .apprxeq.cos.sup.2 (.mu.VNI+(.pi./2))P.sub.0
where P.sub.1 and P.sub.2 are the light intensity signals detected respectively in the output ports of the coupler, and P.sub.0 is the light intensity signal injected in the Sagnac interferometer.
Thus, the amplitude of the current can be known by measuring light intensity signals reflected back in the output ports of the coupler. If the light beams follow similar reciprocal paths, the phase difference depends only upon the Faraday effect. In practice, the light beams do not always follow similar reciprocal paths. Consequently, the phase difference does not depend only upon the Faraday effect. As a matter of fact, the birefringence present in the optical fibre may result in guiding the two light beams into different paths. Because of that, the phase difference between the polarization components of the light beams is affected. Also, as the birefringence is sensitive to temperature variations and mechanical vibrations, the output signal will be unstable. The polarization and the phase difference between the two counter-propagating light beams are independent from the birefringence only if the two light beams follow similar reciprocal paths. It would be possible to obtain an interferometric current sensor perfectly insensitive to mechanical vibrations and to temperature variations only if the two counter-propagating light beams follow similar reciprocal paths.
In the International PCT application NO. WO 93/13428 of CLARKE there is described a Sagnac interferometer for measuring electrical current. CLARKE proposes the use of a high-birefringence spun fibre with a Sagnac interferometer. The object of the CLARKE patent application is to reduce the influence of temperature variations on electrical current readings. This configuration exploits the reciprocity of circular and linear birefringences. This reciprocity is obtained only if the light beam circulating in the clockwise direction along the Sagnac loop and the light beam circulating in the counter-clockwise direction along the Sagnac loop follow similar reciprocal paths. In order to obtain that, the angle between the polarization axes of the high-birefringence spun fibre sections and the optical fibres connected to the coupler must be carefully adjusted so that the counter-propagating light beams follow similar reciprocal paths. Also, the optical fibres connecting the coupler to the high-birefringence spun fibre sections must have a negligible birefringence. Thus, as long as the birefringence of optical fibres connecting the coupler to the Hi-Bi spun fibre is not affected by temperature variations, the configuration proposed by CLARKE provides reciprocal paths with respect to temperature, and therefore a good stability with respect to temperature can be obtained. But, as for stability with respect to mechanical vibrations, CLARKE proposes no solution to this problem. As a matter of fact, in presence of mechanical vibrations, the high-birefringence spun fibre sections and the optical fibres connected to the coupler are affected in a non-uniform manner. Because of that, the paths of the counter-propagating light beams are partially non-reciprocal and therefore the whole system is sensitive to mechanical vibrations. Because of that, a part of the light energy which varies with respect to mechanical vibrations, does not interfere according to the phase difference induced by the Faraday effect, the resulting signal is then unstable. The configuration proposed by CLARKE does not solve the problems related to mechanical vibrations.
certain interferometric current sensors of this kind use active elements such as phase and amplitude modulators, outside and/or inside the Sagnac loop to reduce the sensitivity of the sensor to ambient conditions. But, the complexity of the whole system is increased.
Also known in the art, there are the following patents and articles relating to different apparatuses using optical fibres for measuring different parameters:
U.S. Pat. Nos. 4,370,612 4,542,338 4,545,682 4,634,852 4,773,758 4,848,910 4,949,038
U.K. Patent Application No. 2,251,940
Publications:
Low-Drift Fibre Gyro Using A Superluminescent Diode of Bohm et al., ELECTRONICS LETTERS, May 14, 1981, Vol. 17, No. 10, pages 352-353; PA1 Electric Current Sensors Employing Spun Highly Birefringent Optical Fibers, Richard I. LAMING et al., Journal of Lightwave Technology, Vol. 7, No. 12, December 1989, pages 2084 to 2094; PA1 Degree of Polarization in the Lyot Depolarizer, William K. BURNS, Journal of Lightwave Technology, Vol. LT-I, No. 3, September 1983, pages 475 to 479; PA1 Temperature-Stable Spun Elliptical-Core Optical-Fiber Current Transducer, Ian G. CLARKE, Optics Letters, Vol. 18, No. 2, Jan. 15, 1993, pages 158 to 160; PA1 Development of a Fiber Optic Current Sensor for Power Systems, Trevor W. MacDOUGALL et al., Proceedings of 1991 IEEE Power Engineering Society Transmission and Distribution Conference, pages 336 to 341; PA1 Magnetic Field Sensitivity of Depolarized Fiber Optic Gyros, J. BLAKE, SPIE, Vol. 1367, Fiber Optic and Laser Sensors VIII (1990), pages 81 to 86; PA1 Fiber-Optic Gyroscopes with Depolarized Light, William K. BURNS et al., Journal of Lightwave Technology, Vol. 10, No. 7, July 1992, pages 992 to 999; PA1 Faraday Effect Sensors: The State of the Art, G. W. DAY et al., SPIE, Vol. 985 Fiber Optic and Laser Sensors VI (1988), pages 138 to 150; PA1 Faraday Effect Current Sensing Using a Sagnac Interferometer with a 3.times.3 coupler, presented at Optical Fibre Sensors Conference 1990, R. R. VESSER et al.; PA1 Optical Current Transducers for Power Systems: A review by the Emerging Technologies Working Group for presentation at IEEE/PES 1994 Winter Meeting, pages 1 to 11; PA1 The Fiber-Optic Gyroscope, Herve LEFEVRE, published by Artech House (Boston), pages 58 to 62, 73 to 86, 93 to 101; and PA1 A Magneto-Optic Current Transducer, T. W. CEASE et al., IEEE Transactions on Power Delivery, Vol. 5, No. 2, April 1990, pages 548 to 555. PA1 a light source for generating a light beam; PA1 a single-mode birefringent waveguide having a linear birefringence and a circular birefringence, said circular and linear birefringences having respectively given orders of magnitude, the order of magnitude of said circular birefringence being equal or higher than the order of magnitude of said linear birefringence, said waveguide having a portion for encircling said electrical current; PA1 a beamsplitter having a first input port optically coupled to said light source, and second and third input/output ports optically coupled to ends of said waveguide for launching counter-propagating light beams into the respective ends of the waveguide and for receiving said counter-propagating light beams therefrom; PA1 a pseudo-depolarizer optically coupled in series with said waveguide for converting each of said counter-propagating light beams into a predetermined ratio of useful counter-propagating light signals and useless counter-propagating light signals; and PA1 an optical detector optically coupled to the ends of said waveguide via said beamsplitter for detecting a light intensity, said light intensity resulting from an interference of useful counter-propagating light signals and useless counter-propagating light signals, whereby said light intensity is representative of said current. PA1 a) encircling said electrical current by means of a loop portion of a single-mode birefringent waveguide, said waveguide having a linear birefringence and a circular birefringence, said circular birefringence and linear birefringences having respectively given orders of magnitude, the order of magnitude of said circular birefringence being equal or higher than the order of magnitude of said linear birefringence; PA1 b) generating a light beam; PA1 c) splitting said light beam to launch two counter-propagating light beams into respective ends of said waveguide; PA1 d) pseudo-depolarizing said two counter-propagating light beams launched into said waveguide to convert each of said counter-propagating light beams into a predetermined ratio of useful and useless light signals; PA1 e) interfering said useful counter-propagating light signals and useless counter-propagating light signals to produce an interfered light signal; and PA1 f) optically detecting a light intensity of said interfered light signal, whereby said light intensity is representative of said current.